zulip/static/js/watchdog.js

const unsuspend_callbacks = [];
let watchdog_time = Date.now();

// This field keeps track of whether we are attempting to
// force-reconnect to the events server due to suspecting we are
// offline.  It is important for avoiding races with the presence
// system when coming back from unsuspend.
let suspect_offline = false;

export function set_suspect_offline(suspected) {
    suspect_offline = suspected;
}

export function suspects_user_is_offline() {
    return suspect_offline;
}

/*
    Our watchdog code checks every 5 seconds to make sure that we
    haven't gone 20 seconds since the last "5-second-ago" check.
    This sounds confusing, but it is just is a way to detect that
    the machine has gone to sleep.

    When we detect the condition we call back to server_events code
    to reset ourselves accordingly.
*/

export function check_for_unsuspend() {
    const new_time = Date.now();
    if (new_time - watchdog_time > 20000) {
        // 20 seconds.
        // Defensively reset watchdog_time here in case there's an
        // exception in one of the event handlers
        watchdog_time = new_time;
        // Our app's JS wasn't running, which probably means the machine was
        // asleep.
        for (const callback of unsuspend_callbacks) {
            callback();
        }
    }
    watchdog_time = new_time;
}

export function on_unsuspend(f) {
    unsuspend_callbacks.push(f);
}

setInterval(check_for_unsuspend, 5000);